I'm no expert, but as far as I know there is no other way on Itch Io to have your game play in a browser and be downloadable at the same time as there is currently.
If format is the issue it wouldn't be just an html file, but instead a zip with the html and then the assets in their own folders inside.
Game size may be another possible reason why downloads aren't possible (itch . io has an approx 1-2 GB size limit.)
But I do see games with both options, so it is possible to have both.